In the operation of bulk material conveyors, a belt scraper assembly is provided for removing adherent material from the conveyor belt and depositing it into a discharge area. In the absence of a cleaning device, or as a result of a poorly functioning belt scraper, carry-over spillage material will be accumulated beneath the conveyor belt. In addition to constituting a nuisance, a large amount of valuable product will be deposited beneath the conveyor and may build up sufficiently to interfere with operation of the conveyor. For example, a volume of spillage material 1/16 inch (1.6 mm) deep and 1 inch (25 mm) wide will produce approximately 21/2 cubic feet (0.075 cubic meters) of carry-over per 100 feet per minute (0.48 meters per second) of belt speed each hour. At a conveyor speed of 500 feet per minute (2.5 meters per second), this small stream produces approximately 10 tonnes of carry-over spillage every eight hours, based on a density of 100 pounds per cubic foot (1500 kg/cubic meter).
Consequently, there is considerable interest in improving the operation and efficiency of conveyor belt cleaning devices.
2. Description of the Prior Art
Conventional conveyor belt cleaning devices generally include one or more blades disposed in a plane transverse to the conveyor belt and are urged toward the belt so as to cause engagement of an edge of the blade against the belt surface. Such conveyor belt cleaning devices must be located rearwardly of the head pulley drum on the return path of the conveyor belt so that sufficient flexibility in the belt is provided to permit transverse movement thereof when protruding objects pass the cleaning device. The belt cleaning device is hampered constantly by obstructions such as mechanical fasteners moving at high speeds, which are often driven by hundreds of horsepower.
One of the major causes of failures in conveyor belt cleaning equipment is reverse operation of the conveyor belt. If the belt reverses, the reverse movement of the belt can drive the blades into the belt and cause tearing of the belt and damage to the scraper. This problem occurs frequently on inclined conveyors where the hold back allows the belt to reverse a few inches before locking. It also occurs when a traveling tripper is moved forward when the belt is stopped. A reversing belt condition can occur in the operation of reversible conveyors, single direction conveyors having a "traveling tripper", with the belt reversing over the pulley if the tripper is moved forward while the belt is stopped, and in the operation of single direction conveyors having a telescoping discharge section.
A common situation in which belt damage occurs frequently is in an inclined conveyor which is shut down with a load on the belt. The load tends to pull the conveyor in a reverse direction, sliding downhill. To prevent the belt from running away downhill with a load on it, a device called a "hold-back" is incorporated either onto the shaft that drives the pulley or within the gearing. The hold-back device does not operate with 100% efficiency, and it is common for the belt to creep backward. A very short distance of creep back or roll back, for example, 1/8 inch of roll back, is sufficient to impose tremendous loads on the scraper blades since the scraper is usually working at an angle the belt. Such compression loads will damage the weakest link first, with the weakest link usually being the belt scraper. If the belt scraper is solid and strong enough, it will cause tearing damage to the conveyor belt. Because of this condition, most belt scrapers are designed to engage an unsupported section of the belt after it has left the pulley. In such arrangements, the belt does not have a solid backing, and the belt itself can lift, thereby relieving the forces that would be imposed on the scraper in response to a reversing condition.
Various improved belt scrapers have been proposed or are now in use for engagement against a supported section of the conveyor belt, for example, see U.S. Pat. Nos. 4,349,098; 4,498,577; 4,662,507; and 4,821,867 by Willem Dirk Veenhof. According to those arrangements, a flexible belt scraper assembly is positioned beneath the overhang defined by the discharge pulley drum. The foregoing flexible scraper assemblies have proven effective and efficient in cleaning conveyor belt surfaces in a wide variety of applications. However, severe tension loads may be induced within the flexible scraper strands under certain load conditions which can cause accelerated wear of scraper components and failure.
Some of the means that have been used or are presently being used to permit yieldable engagement of a flexible scraper against the conveyor belt in the pulley drum overhang region is a mounting frame which is counterweighted or spring-loaded to apply yieldable pressure against the belt. Extensive testing and operational experience have shown that the counterweight and spring bias arrangements are subject to damage from the reaction forces transmitted to the support assembly which tend to cause its destruction, and the scraper itself may damage the conveyor belt during reverse operation.
Consequently, there is considerable interest in providing a belt scraper which can operate effectively and efficiently against a drum supported conveyor belt, and which includes means for automatically yielding to prevent damage to the scraper or to the conveyor belt in response to a reversing condition.
It has been determined that the ideal scraper location for maximum efficiency is beneath the overhang defined by the discharge pulley drum, with the scraper engaging the conveyor belt surface which is supported by the face of the pulley drum. Moreover, because of mounting space limitations and the potential for accumulation of load material on exposed scraper components, it is desirable to have the scraper mounting framework rigidly mounted on the pulley drum support frame, with the exposed structure of the scraper assembly being supported beneath the overhang of the pulley drum to minimize exposure to discharged load material. Since the conveyor belt itself is rigidly fit into contact against the cylindrical face of the pulley drum at the point at which scraping and cleaning occur, it is essential that the scraper be modified to enable it to accept a reversing condition without causing damage to the conveyor belt, the scraper itself or its supporting structure.